The present invention relates to polymeric coated composites for packaging and, in particular, to methods for improving the thermal bonding between dissimilar polymeric coatings.
Polyolefinic materials such as low density polyethylene (LDPE), polypropylene (PP), polybutylene (PB) and polystyrene (PS) are used in high volumes by the packaging industry as one component of a laminate composite film for coating substrates such as paper, metal foils, nonwoven fabrics and the like. Such polyolefinic materials have substantially non-polar characteristics and thus are not typically well suited for thermal adhesion to polymeric materials having more polar characteristics.
Methods for making polymeric coated paper and paperboard substrates for use as containers and cartons are well known. See for example, U.S. Pat. Nos. 4,698,246; 4,701,360; 4,789,575; 4,806,399; 4,888,222; and 5,002,833.
In many circumstances, it is desirable to use a non-polar polymeric material on one surface of a substrate for a carton or container and a more polar or otherwise dissimilar polymeric material on another surface. During the process of container construction, the finished package often contains an overlap joint. The overlap joint may be bonded with an adhesive, however it is preferable in many cases to thermally seal the joint by mating the two surfaces under heat and pressure so that they are, in effect, fused together. Thermally bonded surfaces offer the potential of providing greater or more uniform bonding strength than other bonding techniques so that fewer joint failures occur. Difficulties often arise, however, when attempting to obtain a strong bond between dissimilar polymeric materials by the use of thermal bonding techniques alone. Adhesives, on the other hand, may not be suitable in many situations, particularly in food or medical applications where contamination of the food or medical materials with the adhesive may occur.
It is therefore an object of the invention to provide an improved polymer coated substrate for packaging.
Another object of the invention is to provide a method for improving the thermal bonding between dissimilar polymeric materials.
A further object of the invention is provide a method for thermally bonding dissimilar polymeric materials on a coated substrate.
A still further object of the invention is to increase the thermal bonding strength between dissimilar polymeric materials without the need to use a separate or discrete polymeric tie layer between the dissimilar polymeric materials.
Another object of the invention is to improve the thermal bonding strength between dissimilar polymeric materials without the need for elaborate or complicated coating techniques.